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Journal of the Optical Society of America B

Measurement of the four-photon detachment cross section of Cl^− at 1064 μm

Christophe Blondel, Rusty P. Trainham

Published Oct 1, 1989• 17 citations
Publisher

Abstract

An accelerated beam of Cl− ions is photodetached by a single-mode Nd:YAG laser, and the resulting neutrals are detected. By measuring the saturation intensity of this process, one derives the cross section for four-photon detachment. This cross section is smaller by a factor of 6 than a recently calculated value. Comparison of the photodetachment saturation intensity of Cl− with those of other halogen negative ions shows that this corresponds to regular behavior among the halogen elements, suggesting limitations in the theoretical models.

DOI
10.1364/josab.6.001774

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Journal of Physics B Atomic and Molecular Physics•Dec 1, 1972•7 citations
An accelerated beam of Cl− ions is photodetached by a single-mode Nd:YAG laser, and the resulting neutrals are detected. By measuring the saturation intensity of this process, one derives the cross section for four-photon detachment. This cross section is smaller by a factor of 6 than a recently calculated value. Comparison of the photodetachment saturation intensity of Cl− with those of other halogen negative ions shows that this corresponds to regular behavior among the halogen elements, suggesting limitations in the theoretical models.
A polarization dependence of a two-colour two-photon detachment cross section has been observed and calculated for the first time. In a Penning ion trap, electrons were detached from Cl - by an intense infrared laser beam at 1064 nm and a weak ultraviolet laser beam at 343 nm. Both beams were circularly polarized. The ratio of the two-colour two-photon cross section for photons with the same circular polarization to that for photons with opposite circular polarization was measured to be 1.085 +0.012 -0.007 . Also presented is a simple calculation of the ratio, the result of which is consistent with the observation.
Photodetachment from negative ions in a static electric field has been studied using ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{S}}^{\mathrm{\ensuremath{-}}}$. These experiments were done with greater precision than previous studies and allow a quantitative comparison of data with theory. A 10-keV ion beam was sent through a region with fields of up to 1.5 kV/cm applied parallel to the beam velocity. A pulsed dye-laser beam, perpendicular to the ion beam, photoneutralized the ions. The relative cross sections were measured by detecting the resulting fast neutral atoms. Detachment below threshold and oscillations on the cross section above threshold were observed near the ${\mathrm{S}}^{\mathrm{\ensuremath{-}}}$ threshold at 16 269.5 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and near the ${\mathrm{Cl}}^{\mathrm{\ensuremath{-}}}$ threshold at 29 138.3 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The phase of the oscillations in the data is in good agreement with predictions for s-wave photodetachment in a static electric field, but the amplitude of the oscillations is found to be slightly reduced.
Multiphoton excitation with elliptically polarized light brings out more information in the photodetachment or photoionization processes than linearly polarized light. This information can be reached by recording photoelectron angular distributions for several values of the ellipticity angle, and fitting the whole set of data with an analytic formula. We give here the appropriate formula, valid for any number N of exciting photons, with the assumptions of (1) an initially even and momentumless (J equals 0) atom or ion and (2) that after excitation of an electron, the residual core be left in an odd state. These are the conditions met by multiphoton detachment from halogen negative ions. With elliptically polarized light of (lambda) equals 1064.2 nm, we have measured the angular distributions of three-photon detached electrons from the ion I-, and four-photon detached electrons from Cl- and F-. The experimental results are quantitatively analyzed and commented on.
The cross sections for one-, two-, and three-photon detachment from fluorine negative ions have been measured using light at the fundamental and harmonic frequencies of a 1064-nm pulsed neodymium-doped yttrium aluminum garnet laser. The ions were stored in a Penning ion trap, the depletion due to illumination was measured, and the data were fitted to a model for stored ions illuminated by pulsed light. The values obtained for the detachment cross sections are ${\ensuremath{\sigma}}^{(1)}$=2.8(5)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}18}$ cm2{\mathrm{cm}}^{2}cm2, ${\ensuremath{\sigma}}^{(2)}$=2.0(7)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}50}$ cm4{\mathrm{cm}}^{4}cm4 sec, and ${\ensuremath{\sigma}}^{(3)}$ ${=7.9}_{\mathrm{\ensuremath{-}}3.6}^{+6.6}$\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}83}$ cm6{\mathrm{cm}}^{6}cm6 sec2{\mathrm{sec}}^{2}sec2. These values are compared with theoretical calculations and other experimental results.